Photovoltaic devices (e.g., solar cells) are known in the art. A solar cell may include, for example, a photoelectric transfer film made up of one or more layers located between a pair of substrate. These layers may be supported by a glass substrate. Example solar cells are disclosed in U.S. Pat. Nos. 4,510,344, 4,806,436, 6,506,622, and 5,977,477, the disclosures of which are hereby incorporated herein by reference.
Substrate(s) in a solar cell are sometimes made of glass. Glass that is fairly clear in color and highly transmissive to visible light is sometimes desirable. Glass raw materials (e.g., silica sand, soda ash, dolomite, and/or limestone) typically include certain impurities such as iron, which is a colorant. The total amount of iron present is expressed herein in terms of Fe2O3 in accordance with standard practice. However, typically, not all iron is in the from of Fe2O3. Instead, iron is usually present in both the ferrous state (Fe2+; expressed herein as FeO, even though all ferrous state iron in the glass may not be in the form of FeO) and the ferric state (Fe3+). Iron in the ferrous state (Fe2+; FeO) is a blue-green colorant, while iron in the ferric state (Fe3+) is a yellow-green colorant. The blue-green colorant of ferrous iron (Fe2+; FeO) is of particular concern when seeking to achieve a fairly clear or neutral colored glass, since as a strong colorant it introduces significant color into the glass. While iron in the ferric state (Fe3+) is also a colorant, it is of less concern when seeking to achieve a glass fairly clear in color since iron in the ferric state tends to be weaker as a colorant than its ferrous state counterpart.
It has been found that the use of a low-iron highly transparent (optionally patterned) glass is advantageous for solar cell applications. The use of the low-iron composition in combination with the patterned surface(s) of the glass substrate(s) has been found to be advantageous with respect to optical properties, thereby leading to increased solar efficiency of a solar cell.
A limiting performance factor for glass in connection with photovoltaic devices is the seed level, as opposed to solar performance, in certain instances. High seed levels are undesirable, in that these represent small imperfections in the glass. Thus, it would be desirable to provide a glass composition, and method of making glass, in which seed level can be reduced (or seed level refining time can be reduced), for use in photovoltaic devices or the like.
In certain example embodiments of this invention, a solar cell glass substrate has a visible transmission of at least 75% (more preferably at least 80%, even more preferably at least 85%, and most preferably at least about 90%). In making such a glass, a batch therefor includes a base glass (e.g., soda lime silica glass) and in addition comprises (or consists essentially of in certain other embodiments) a very small amount of total iron.
It has been found that the use of lithium oxide in connection with high transmission low iron glass has the effect of improve refining conditions by lower the melting temperature of the glass (log η=2). In particular, it has been found that providing appropriate amounts of lithium oxide in low-iron high transmission glass lowers the log η=2 temperature so as to improve the refining of these oxidized glasses. Thus, the refining time required to achieve a seed free glass (or substantially seed free glass) can be reduced, which is high advantageous with respect to the glass manufacturing process. In certain example embodiments, the glass and glass batch may include antimony oxide in order to support oxidation of the FeO to Fe2O3.
In certain example embodiments of this invention, the low iron glass is particularly efficiently made with respect to both performance and manufacturability using a combination of certain amounts of salt cake, antimony oxide (e.g., antimony trioxide), optionally sodium nitrate, and lithium oxide.
In certain example embodiments, the resulting patterned glass substrate may have fairly clear color that may be slightly yellowish (a positive b* value is indicative of yellowish color). For example, in certain example embodiments, the patterned glass substrate may be characterized by a visible transmission of at least 90%, a total solar/energy value of at least 90%, a transmissive a* color value of from −1.0 to +1.0 (more preferably from −0.5 to +0.5, and most preferably from −0.2 to 0), and a transmissive b* color value of from 0 to +1.5 (more preferably from +0.1 to +1.0, and most preferably from +0.2 to +0.7). These properties may be realized at an example non-limiting reference glass thickness of from about 3-4 mm.
In certain example embodiments of this invention, there is provided a method of making patterned glass, the method comprising: providing a molten glass batch in a furnace or melter comprising SiO2, from about 0.01 to 0.06% total iron, salt cake, lithium oxide, and antimony oxide, and refining the glass batch wherein the batch has a seed free time of less than 100 minutes; forwarding a glass ribbon from the furnace or melter to a nip between first and second rollers, at least one of the rollers having patter defined in a surface thereof, wherein the glass ribbon reaches the nip at a temperature of from about 1,900 to 2,400 degrees F.; at the nip, transferring the pattern from the roller(s) to the glass ribbon; the glass ribbon being at a temperature of from about 1,100 to 1,600 degrees F. upon exiting the nip; and annealing the glass ribbon at least after the ribbon exits the nip, thereby providing a patterned glass having a visible transmission of at least 90%, from about 0.01 to 0.06% total iron, from about 0.25 to 3.5% lithium oxide, from about 0.3 to 0.4% salt cake, and from about 0.01 to 1.0% antimony oxide.
In certain other example embodiments, there is provided a method of making a soda-lime-silica based low-iron highly transmissive glass for use in a photovoltaic device, wherein the glass has visible transmission of at least 90%, a transmissive a* color value of −1.0 to +1.0 and a transmissive b* color value of from 0 to +1.5; and wherein the method comprises providing the lithium oxide, antimony oxide and salt cake in the recited amounts in a batch in making the low-iron glass so that a seed free time in making the glass is no more than 100 minutes.